The present invention relates to a wireless local area network (WLAN) and, more particularly, to a method and apparatus for correlating a signal of a WLAN having an extended data rate.
A wireless local area network (WLAN) is a data communication system implemented as an extension of or alternative to a wired data communication network (LAN). A WLAN provides location independent access between computing devices using radio frequency or other wireless communication techniques. WLANs have been or are being developed to conform to a number of standards, including the IEEE 802.11, Bluetooth and HomeRF standards. The IEEE 802.11 standard, INFORMATION TECHNOLOGY—TELECOMMUNICATIONS AND INFORMATION EXCHANGE BETWEEN SYSTEMS—LOCAL AND METROPOLITAN AREA NETWORKS—SPECIFIC REQUIREMENTS—PART 11: WIRELESS LAN MEDIUM ACCESS CONTROL AND PHYSICAL LAYER (PHY) SPECIFICATIONS, Institute of Electrical and Electronics Engineers, was approved in 1997 and a supplement providing for higher data rate WLANs, IEEE 802.11b, WIRELESS LAN MEDIUM ACCESS CONTROL (MAC) AND PHYSICAL LAYER (PHY) SPECIFICATIONS: HIGHER SPEED PHYSICAL LAYER (PHY) EXTENSION IN THE 2.4 GHz BAND, was approved in 1999. The IEEE 802.11 standards define a protocol and a compatible interface for data communication in a local area network via radio or infrared-air transmission. While the standard defines an infrared-air communication interface, radio frequency (RF) communication is the most commonly used communication method for WLAN implementation.
The IEEE 802.11 standard defines the physical layer (PHY) and a media access control (MAC) sublayer for WLANs with data rates of 1 Mbits/s or 2 Mbits/s using either frequency hopping spread spectrum (FHSS) or direct sequence spread spectrum (DSSS) RF communication techniques. These RF systems operate in the 2.4 GHz, ISM (Instrument, Scientific, and Medical) frequency band. As defined by local regulations, the ISM band extends from 2.4000-2.4835 GHz in the U.S., Canada and much of Europe. A similar, if not identical frequency band, is set aside for use by unlicensed RF radiators in several other countries.
For a frequency hopping system, the transmission frequency is periodically shifted in a pseudorandom pattern known to both the transmitter and the receiver. For North America and most of Europe, 79 hop channels of 1 MHz and a maximum channel dwell time of 400 ms are specified for IEEE 802.11 FHSS systems. FHSS permits a simpler transceiver design than that required for a DSSS system. However, high bit packing coding schemes required for reliable operation of FHSS in the narrow channels prescribed by the regulations become impractical at high data rates due to high signal-to-noise ratios. As a result, the data rate of practical FHSS, ISM band systems is relatively limited and DSSS is the technique of choice for higher data rate WLANs.
In a DSSS system, the modulated signal is spread over a transmission bandwidth greater than that required for the baseband information signal by directly modulating the baseband information signal with a pseudorandom noise (PN) or spreading code that is known to both the transmitter and the receiver. Each data bit of the baseband information is mixed with each of a plurality of chips or bits of the spreading code. For example, the spreading code for DSSS wireless networks conforming to the basic IEEE 802.11 standard is a Barker sequence comprising eleven chips having the sequence “01001000111” (non-polar signal) or “+1, −1, +1, +1, −1, +1, +1, +1, −1, −1, −1” (non-return to zero (NRZ) or polar signal). Referring to FIG. 1, each bit of the baseband data 20 is mixed (Exculsive-ORed) 22 with the eleven chips of the Barker sequence 24 to form an 11-chip codeword. A logic “0” baseband information bit is encoded as a first codeword 26 and a logic “1” is encoded as a second chip sequence or codeword 28.
The resulting multi-chip symbol or codeword is transmitted in the bit period of the baseband data bit or the time between the starting and ending of the baseband bit. If the bit rate for baseband information is 1 MSymbols/s, the eleven chip Barker sequence is encoded at a chipping rate of 11 MHz. In a phase shift key (PSK) modulated system like IEEE 802.11 systems, the encoded chips are transmitted as phase changes in the transmitted signal. Since the signal changes phase several times in the period required to transmit a single data bit, the frequency bandwidth must be wider than that required for the baseband. When the signal is demodulated, the frequency spreading is reversed and signals from potentially interfering radiators are eliminated decreasing the likelihood that the signal of interest will be jammed.
IEEE 802.11 compliant DSSS systems utilize differential phase shift keying where the relative phase difference between the waveforms received during to successive codeword intervals indicates the value of transmitted data. Differential binary phase shift keying (DBPSK) (one phase shift per information bit) modulation is used for transmission at the basic data rate of 1 Mbits/s. Differential quadrature phase-shift keying (DQPSK) (four phase shifts to encode two information bits) is used to increase the data transfer rate to 2 Mbits/s. For 2 Mbits/s DQPSK modulation, the information data stream is grouped into pairs of bits or dibits and one of four codewords is selected based on the values of the bits of a dibit. Alternate codewords are multiplied by either a first cosinusoidal phase-shift modulation signal and transmitted as a first “in-phase” (I) signal or a second 90° phase-shifted sinusoidal carrier at the same frequency and transmitted as a second “quadrature” (Q) signal. The 11-chip Barker code and a chip rate of 11 Mchip/s permits three non-overlapping DSSS channels in the ISM frequency band.
The preamble of the IEEE 802.11 data packet is used by the receiver to initiate spreading code synchronization is always transmitted as the DBPSK wave form. This permits all receivers to identify the transmitted waveform and, if the receiver is capable, switch to a higher rate mode of operation for interaction a particular WLAN device. The header of an IEEE 802.11 data packet which includes a cyclic redundancy check code, a packet payload transmission rate indicator, and payload length signal may be transmitted as either a DBPSK or DQPSK waveform.
To achieve higher data rates, the IEEE 802.11b revision adopts Complementary Code Keying (CCK) to replace the 11-chip Barker sequence for modulating data packet payloads. Complementary codes or binary, complementary sequences are polyphase codes comprising a pair of equal finite length sequences having the property that the number of pairs of like elements with any given separation in one series is equal to the number of pairs of unlike elements with the same separation in the second series. As a set, these code sequences have unique mathematical properties that facilitate distinguishing between code words at the receiver even in the presence of substantial noise and multipath interference. For an 11 Mbits/s data rate the information data stream is divided into eight bit segments. The values of six of the data bits are used to generate one of 64 unique subcodes. The values of the two remaining data bits are used to select one of the DQPSK phases for rotating the selected subcode producing 256 possible codewords for transmission. Systems operating in the 5.5 Mbits/s mode use two data bits to generate one of four subcodes and two bits are used to select one of the four DQPSK phases. With a symbol rate of 1.375 Msymbols/s, an eight chip spreading code, and a chipping rate of 11 MHz the high data rate waveform occupies approximately the same bandwidth as that of the 2 Mbits/s DQPSK waveform of the lower rate systems. As a result, the ISM band is sufficiently wide for three non-overlapping higher data rate channels promoting interoperability of the lower and higher data rate systems.
In a receiver, the “as received” analog signal is converted to a digital signal and correlation is used to strip the PN or spreading code from the digital signal. In the CCK modes utilized by the higher data rate systems, a bank of correlators followed by a largest correlation value detector is used to detect the modulation. The CCK codewords are an eight chip Walsh code that can be decoded with a fast Walsh transform. The correlators typically implement the transform as a butterfly function comprising 64 separate correlations requiring 512 complex additions to decode the 64 subcodes which are used to estimate six bits of reconstructed data. The remaining two bits of the signal are demodulated using the DQPSK demodulation. For 5 Mbits/s operation, 28 butterflies and 112 complex additions are required to decode two bits.
In a pending U.S. patent application, the inventor and others have disclosed a method of extending the data rate of a DSSS WLAN through the use of bandwidth efficient M-ary phase shift keying modulation. While the signals of the extended data rate system are structurally similar to those of the higher data rate IEEE 802.11b CCK operating modes, the information bits encode 4096 codewords for transmission. Correlation of the signal utilizing the process of the IEEE 802.11b CCK modes would require a substantial bank of correlators significantly increasing the cost and complexity of the transceiver. What is desired, therefore, is a method of correlating an M-ary PSK waveform that reduces the number of correlators required in a reciever.